Abstract
Cercospora leaf spot (CLS), an economically significant foliar disease of sugarbeet caused by Cercospora beticola, continues to be a challenge for growers. Sugarbeet varieties with high tolerance to CLS with BvCR4-mediated monogenic resistance have been commercially available to sugarbeet growers since 2020. The potential for C. beticola adaptation to these new varieties is suspected to increase with widescale adoption of these varieties. There is limited knowledge on the impact of variety mixtures on CLS epidemiology and the evolution of C. beticola populations; however, variety mixtures have been shown to reduce disease severity in other crops despite unclear effects on preserving durability of resistance genes. A field trial was conducted in 2024 in which a randomized split-plot design with four replications was used. Main plot treatments consisted of two sugar beet varieties, one BvCR4 variety (CLS rating ~2.0) and one moderately susceptible variety (CLS rating ~4.9), planted in pure stand compared to mixtures consisting of either 25:75, 50:50, or 75:25 ratios. Plots were inoculated with a mixture of CLS-infested leaves collected from both BvCR4 varieties and CLS-susceptible varieties in 2023. Fungicide spray programs were assigned as subplots within variety mixtures and consisted of a nontreated control and a reduced fungicide program with one DMI application within a three-spray program. A rigorous sampling strategy was employed to obtain representative single-conidium isolates of C. beticola from the field trial (n = 320). Significant two-way interactions were present for overall CLS severity and sucrose yield, suggesting reduced fungicide programs can be utilized in varietal mixtures as well as pure stands of a BvCR4 variety. By investigating the impact of variety mixtures and reduced fungicide applications on CLS epidemiology, we aim to guide the utilization of sugarbeet varieties with BvCR4-mediated resistance to preserve the durability of the BvCR4 gene and mitigate the prevalence of C. beticola insensitive to DMI fungicides.